1. Field of the Invention
The invention relates generally to the field of electromagnetic geophysical surveying. More specifically, the invention relates to methods for accurately determining the orientation of electromagnetic sensors deployed to perform such surveying.
2. Background Art
Electromagnetic survey systems and methods provide a variety of data about subsurface formations; including, for example, the spatial distribution of resistivity in the Earth's subsurface. Such data are interpreted and evaluated, among other purposes, to improve prediction of oil and gas production from a given reservoir or field, to detect new oil and gas reservoirs, to provide a picture or model of subsurface formations and of reservoirs in order to facilitate the removal of hydrocarbons, and/or to reduce the risk or otherwise enhance the process of well location.
Controlled source electromagnetic (“CSEM”) surveying includes imparting an electric current or a magnetic field into subsurface Earth formations (through the sea floor in marine surveying or through the borehole fluid in borehole surveying), and measuring voltages and/or magnetic fields induced in electrodes, antennas and/or magnetometers disposed near the Earth's surface, on the sea floor, or in a borehole. The voltages and/or magnetic fields are induced in response to the electric current and/or magnetic field imparted into the Earth's subsurface, and the recorded signal is interpreted in terms of distributions of resistivity, induced polarization, etc, within the earth.
Controlled source surveying, as known in the art, typically includes imparting continuous, alternating electric current into the subsurface. The alternating current may have one or more selected frequencies. Such surveying is known as frequency domain controlled source electromagnetic (f-CSEM) surveying. f-CSEM surveying techniques are described, for example, in Sinha, M. C. Patel, P. D., Unsworth, M. J., Owen, T. R. E., and MacCormack, M. G. R., 1990, An active source electromagnetic sounding system for marine use, Marine Geophysical Research, 12, 29-68. Other publications which describe the physics of and the interpretation of electromagnetic subsurface surveying include: Edwards, R. N., Law, L. K., Wolfgram, P. A., Nobes, D. C., Bone, M. N., Trigg, D. F., and DeLaurier, J. M., 1985, First results of the MOSES experiment: Sea sediment conductivity and thickness determination, Bute Inlet, British Columbia, by magnetometric offshore electrical sounding: Geophysics 50, No. 1, 153-160; Edwards, R. N., 1997, On the resource evaluation of marine gas hydrate deposits using the sea-floor transient electric dipole-dipole method: Geophysics, 62, No. 1, 63-74; Chave, A. D., Constable, S. C. and Edwards, R. N., 1991, Electrical exploration methods for the seafloor: Investigation in geophysics No 3, Electromagnetic methods in applied geophysics, vol. 2, application, part B, 931-966; and Cheesman, S. J., Edwards, R. N., and Chave, A. D., 1987, On the theory of sea-floor conductivity mapping using transient electromagnetic systems: Geophysics, 52, No. 2, 204-217. Typical borehole-related applications are described in Strack (U.S. Pat. Nos. 6,541,975 B2, 6,670,813, and 6,739,165) and Hanstein et al., (U.S. Pat. No. 6,891,376). The proposed methodology is not limited to such applications, as it is more general than these specific contexts.
Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying (t-CSEM™). In t-CSEM, electric current is imparted into the Earth at the Earth's surface, in a manner similar to f-CSEM, but in transient fashion. The initial electric current may be direct current (DC). At a selected time, the electric current is switched off, and induced voltages and/or magnetic fields are measured, typically with respect to time over a selected time interval, at the Earth's surface. The switching constitutes the transient event that gives the technique its name; in contrast with certain realizations of f-CSEM (which also involve switching), in t-CSEM a long time interval elapses before the next transient is initiated, long enough for the induced fields to decay away, so that the detection occurs while the source is inactive. The electrical structure of the subsurface is inferred by the time distribution of the induced voltages and/or magnetic fields. t-CSEM techniques are described, for example, in Strack, K.-M., 1992, Exploration with deep transient electromagnetics, Elsevier, 373 pp. (reprinted 1999).
Following are described several patent publications which describe various aspects of electromagnetic subsurface Earth surveying. U.S. Pat. No. 6,603,313 B1 issued to Srnka discloses a method for surface estimation of reservoir properties, in which location of and average earth resistivities above, below, and horizontally adjacent to subsurface geologic formations are first determined using geological and geophysical data in the vicinity of the subsurface geologic formation. Then dimensions and probing frequency for an electromagnetic source are determined to substantially maximize transmitted vertical and horizontal electric currents at the subsurface geologic formation, using the location and the average earth resistivities. Next, an electromagnetic source is activated at or near surface, approximately centered above the subsurface geologic formation and a plurality of components of electromagnetic response is measured with a receiver array. Geometrical and electrical parameter constraints are determined, using the geological and geophysical data. Finally, the electromagnetic response is processed using the geometrical and electrical parameter constraints to produce inverted vertical and horizontal resistivity depth images. Optionally, the inverted resistivity depth images may be combined with the geological and geophysical data to estimate the reservoir fluid and shaliness properties. This method employs a simple technique for determining the orientations of the receivers, which is improved upon with the present invention.
U.S. Pat. No. 6,628,110 B1 issued to Eidesmo et al. discloses a method for determining the nature of a subterranean reservoir whose approximate geometry and location are known. The disclosed method includes: applying a time varying electromagnetic field to the strata containing the reservoir; detecting the electromagnetic wave field response; and analyzing the effects on the characteristics of the detected field that have been caused by the reservoir, thereby determining the content of the reservoir, based on the analysis. This method employs a simple technique for determining the orientations of the receivers, which is improved upon with the present invention.
U.S. Pat. No. 6,541,975 B2 and U.S. Pat. No. 6,670,813 issued to Strack disclose a system for generating an image of an Earth formation surrounding a borehole penetrating the formation. Resistivity of the formation is measured using a DC measurement, and conductivity and resistivity of the formations is measured with a time domain signal or AC measurement. The acoustic velocity of the formation is also measured. The DC resistivity measurement, the conductivity measurement made with a time domain electromagnetic signal, the resistivity measurement made with a time domain electromagnetic signal and the acoustic velocity measurements are combined to generate the image of the Earth formation. In this method, the orientation of the receivers is determined using conventional borehole methods, and the present invention is not applicable to this context.
U.S. Pat. No. 6,739,165 issued to Strack discloses a method where transient electromagnetic measurements are performed with a receiver or transmitter being placed in a borehole and the other being placed on the surface. Either is moved between initiations of the transient source (and the consequent transmission of EM energy with the earth) to new locations where the experiment is repeated. After data processing, images of fluid content changes of the reservoir are obtained. This method employs a simple technique for determining the orientations of the surface receivers, which is improved upon with the present invention.
International Patent Application Publication No. WO 0157555 A1 discloses a system for detecting a subterranean reservoir or determining the nature of a subterranean reservoir whose position and geometry is known from previous seismic surveys. An electromagnetic field is applied by a transmitter on the seabed and is detected by antennae also on the seabed. A refracted wave component is sought in the wave field response, to determine the nature of any reservoir present. This method employs a simple technique for determining the orientations of the receivers, which is improved upon with the present invention.
International Patent Application Publication No. WO 03048812 A1 discloses an electromagnetic survey method for surveying an area previously identified as potentially containing a subsea hydrocarbon reservoir. The method includes obtaining first and second survey data sets with an electromagnetic source aligned end-on and broadside relative to the positions of the same or different receivers. The invention also relates to planning a survey using this method, and to analysis of survey data taken in combination, which allows the galvanic contribution to the signals collected at the receiver to be contrasted with the inductive effects, and the analysis of the effects of signal attenuation, which are highly dependent on local properties of the rock formation, overlying water, and air at the survey area. This is very important to the success of using electromagnetic surveying for identifying hydrocarbon reserves and distinguishing them from other classes of subsurface structure. This method employs a simple technique for determining the orientations of the receivers, which is improved upon with the present invention.
U.S. Patent Application Publication No. 2004/232917 filed by Wright et al. relates to a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (MTEM) measurements on or near the Earth's surface using at least one source, means for measuring the system response, and at least one receiver for measuring the resultant earth response. All signals from the or each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The system and method enable subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored. This method employs a simple technique for determining the orientations of the receivers, which is improved upon with the present invention.
U.S. Pat. No. 5,467,018 issued to Rueter et al. discloses a bedrock exploration system. The system includes transients generated as sudden changes in a transmission stream, which are transmitted into the Earth's subsurface by a transmitter. The induced electric currents thus produced are measured by several receiver units. The measured values from the receiver units are passed to a central unit. The measured values obtained from the receiver units are digitized and stored at the measurement points, and the central unit is linked with the measurement points by a telemetry link. By means of the telemetry link, data from the data stores in the receiver units can be successively passed on to the central unit. This method employs a simple technique for determining the orientations of the receivers, which is improved upon, in the marine context, with the present invention.
U.S. Pat. No. 5,563,913 issued to Tasci et al. discloses a method and apparatus used in providing resistivity measurement data of a sedimentary subsurface. The data are used for detecting and mapping an anomalous resistivity pattern. The anomalous subsurface resistivity pattern is associated with and an aid for finding oil and/or gas traps at various depths down to a basement of the sedimentary subsurface. The apparatus is disposed on a ground surface and includes an electric generator connected to a transmitter with a length of wire with grounded electrodes. When large amplitude, long period, square waves of current are sent from a transmission site through the transmitter and wire, secondary eddy currents are induced in the subsurface. The eddy currents induce magnetic field changes in the subsurface which can be measured at the surface of the earth with a magnetometer or induction coil. The magnetic field changes are received and recorded as time varying voltages at each sounding site. Information receiver, and resistivity variations of the subsurface formations are deduced from the amplitude and shape of the measured magnetic field signals plotted as a function of time after applying appropriate mathematical equations. The sounding sites are arranged in a plot-like manner to ensure that aerial contour maps and cross sections of the resistivity variations of the subsurface formations can be prepared. In this method, the orientation of the receivers is determined using conventional land-survey methods, and the present invention is not applicable to this context.
Other patents related to t-CSEM surveying include U.S. Pat. No. 7,388,382 issued to Strack et al., U.S. Pat. No. 7,356,411 issued to Stoyer et al., U.S. Pat. No. 7,328,107 issued to Strack et al. and U.S. Pat. No. 7,340,348 issued to Strack et al. all of which are assigned to the assignee of the present invention.
Many of the foregoing electromagnetic survey techniques are performed by deploying an array of electric and/or magnetic field sensors on the bottom of a body of water. For purposes of accurately mapping geologic structures using such electromagnetic survey techniques, it is normally important to be able to determine the geodetic orientation of the individual sensors as well as their geodetic positions. Techniques for determining geodetic orientation may include providing directional sensing devices for each sensor. Other techniques include measuring relative amplitudes of electromagnetic signals in each of two or three mutually orthogonal directions and using the geodetic positions of the electromagnetic signal source and receiver as a reference for geodetic orientation of the signals. The latter techniques have the advantage of eliminating the need to provide directional sensing devices for each electromagnetic sensor, which in large sensor arrays can be cost prohibitive and unreliable because of the number of sensors. However, the latter techniques can be inaccurate because a simplifying assumption made in determining signal direction is that the electromagnetic energy propagates in the vertical plane which includes both source and receiver, and that the polarization direction is orthogonal to the propagation direction. Because electrical conductivity in the subsurface is not uniform, such assumption is not precise; instead the energy propagation may depart from this vertical plane. What is needed is a method for determining electromagnetic sensor orientation that uses electromagnetic signal propagation direction yet avoids the inaccuracy associated with electromagnetic wave propagation through the subsurface, which may lie outside of this vertical plane.